@Jaguares4real No, it's not cheating and calling someone like Heinz a turncoat isn't justified at all.
There are three points of view to look at this. That of the player, the (selectors of the) national team that calls up the player to represent the country, and World Rugby.
Player
Assuming every player would like to represent his country, he is bound by the eligibility rules set by others. Reasons for aspiring to play test rugby can vary, and include reasons like national pride, representing family and heritage, and even financial reasons. They're all valid (yes, financial reasons, too; it's a professional sport and careers are relatively short compared to other professions). I'm not saying that I like it when players play for a certain national team for financial reasons, by the way.
I have read an interview with Heinz after he got selected and it was a nice story about his English grandmother and wanting to make her proud, and totally believable. He was just as surprised about his selection as many others, but I can't see any reason for why he should not accept the invitation from the selectors. As for players with financial reasons to accept an invitation to play for a national team: nobody would say no to a big bonus from his employer, or would you?
National selectors
The national team selectors should select the best possible players that are eligible. You can question that decision in this case, but I don't know anything about the current English halfback pool to know whether there are better (fit) halfbacks available. I don't think national selectors have to make the decision of whether players who weren't born in the country or developed their rugby skills and experience in another country, should or should not be selected (except maybe for the so-called "project players" and targeted, intentional poaching). They simply have to respect the eligibility criteria and then pick the players that best fit the profile of the player they're looking to select for their team.
World Rugby
And then there's World Rugby, that sets the eligibility criteria. If someone is to blame for national teams selecting players who weren't born in the country or developed their rugby skills and experience in another country, it's the organisation that sets the rules. And WR has to consider all the options and considerations pro and contra.
As @Catogrande rightly pointed out, in the case of some small countries (including the Pacific Nations) there is something to say for the grandparent rule, because otherwise it may be hard for emigration countries (i.e. countries with a net emigration, as opposed to immigration countries with a net immigration) to select a team of quality players. Migration, for whatever reason, is an important reality; certainly for countries that are former colonies or for poor countries.
Migration means that ties to new countries are established, but doesn't necessarily mean that ties to the old country will be cut. It has always been that way, and probably always will. National or cultural heritage plays a bigger role in some cultures (e.g. the Pacific cultures) than in others; some feel very strong ties to the country in which their grandparents were born. It's something that needs to be respected.
World Rugby has a difficult job to set eligibility rules in this reality; maybe, one day the grandparent rule will disappear; or maybe they'll limit its application to certain nations (good luck with that!). They already have restricted the residency rule from 3 to 5 years. But it's World Rugby where the responsibility lies; not the player, not the national team selectors.
So blaming a player for accepting an invitation to represent the national team of the country in which his grandparent was born is silly. Calling him a turncoat is ridiculous if you can't prove that that player moved to the country of his grandparent with the sole or main objective of making the national team of that country. I think it's fair to say that making the England team wasn't the reason for Heinz signing a contract with an English club and moving there.